The techniques for marking or patterning flat substrates by laser ablation using beam scanners and focussing lenses is extremely well known and many different arrangements for carrying out this operation are used.
The lasers used cover almost all lasers commonly available with wavelengths ranging from the (deep ultra violet) DUV at 193 nm to the (far infra-red) FIR at 10.6 μm, with pulse lengths ranging from the femto-second range to continuous (CW) operation and with average powers ranging from the fraction of a Watt level to many hundreds of Watts.
Laser beam scanner units are commonly based on dual axis oscillating mirrors driven by galvanometer or other motors where the requirement is to mark or pattern over a two dimensional area. For the case where scanning in one axis only is required rotating polygonal mirrors are often used.
A variety of different lenses are used to focus the beam onto the substrate surface. These can range from simple singlet lenses to complex multi-element lenses. The lenses can be located either before or after the scanner unit. For the case where it is situated after the scanner unit, a lens of telecentric type is often used.
A common feature of all these scanning optical systems is that the quality of the focal spot generated on the surface of a flat substrate at off-axis points away from the centre of the scan field is always inferior in terms of minimum size and optimum shape to that generated on axis in the centre of the scan field. These off-axis focal spot distortion effects are due to aberrations induced in the laser beam as it passes at an angle through the scan lens elements. The distortion effects get significantly worse at the extremes of the lens scan field.
One major scan lens aberration is that of field curvature. In this case the smallest focal spots obtainable at off axis points occur at different distances from the lens compared to that for an on axis point. This means that the focal spot formed at an off axis point on a flat substrate is of different diameter to that formed on axis leading to variations in the power and energy density over the scan field. This aberration is readily correctable by the addition of an extra axis to the scanner unit in the form of a dynamically controlled variable telescope. This unit varies the collimation of the beam and causes the beam entering the lens to diverge or converge such that the distance of the focal spot from the lens can be controlled. By this method, the optimum beam focal spot can be arranged to coincide exactly with the surface of a flat substrate at all points over the scan field right up to the extreme edges. Such field flattening techniques are well known and suitable equipment, able to dynamically correct for field curvature, is readily available.
However, other serious off-axis lens aberration effects exist that are not so readily correctable. These are the aberrations that lead to distortion of the focal spot shape as the beam moves from the centre of the field to the extremes. In their very simplest form, these aberrations lead to an increase in the diameter of the Gaussian profile focal spot at the field edges compared to the field centre. In their more usual form, however, these aberrations cause a distortion of the shape of the off axis focal spots with the formation of a comet-like tail. The power or energy density distribution may depart very significantly from a Gaussian distribution. The main effect of these focal spot distorting aberrations is to spread the laser beam power or pulse energy over a larger area and hence reduce the peak power or peak pulse energy density in the off-axis focal spots compared to those in a focal spot on axis.
The process of laser ablation of materials generally has a well defined threshold in terms of laser power or energy density and hence the width of any line structure ablated in a thin film or layer of material depends on the diameter of the focal spot at a power or energy density level equal to the ablation threshold. Hence, any lens aberrations that give rise to a spreading of the laser power or energy over a larger area or a departure from a Gaussian distribution with an enhanced level of energy or power in the wings of the distribution will cause a reduction in the peak power or energy density in the focal spot and can cause a change in the diameter of the focal spot at the level set by the ablation threshold and a corresponding change in the width of the line structure ablated. This line width change can be an increase or a decrease depending on the level of the threshold for ablation compared to the peak power or energy density. In the worst case, where the ablation threshold power or energy density level is close to the peak value and the process window in terms of allowed variation from maximum to minimum power or energy density is small, then any significant reduction in the peak power or energy density in the focal spot can cause the peak level to drop below the ablation threshold and no line will be ablated.
For high resolution line structures in thin and thick film based functional devices such as printed circuit boards, touch screens, displays, sensors, solar panels and other micro-electronic devices, accurate control of the width of the ablated structure is of paramount importance to ensure reliable operation. In this case, a method for overcoming the off axis uncorrectable lens aberrations is required. Adding more elements to the lens reduces the off axis distortion effects and can significantly improve the lens performance but such a solution significantly increases system complexity and cost and does not completely solve the problem.
The invention described herein seeks to provide an alternative way to compensate for the off axis, focal spot distorting aberrations found in standard scan lenses.